1. Field of the Invention
The present invention relates to a method of and a system useful for exposing plural images on a photosensitive material with a photocomposer.
2. Description of the Prior Art
A photocomposer is an apparatus well known in the art for exposing plural images of an original film on a photosensitive material, such as a presensitized plate. In photocomposing operation, the arrangement of the images is planned so that as many images as possible can be printed with a single printing plate made from the photosensitive material.
FIG. 1A is a diagram showing a printing plate on which plural images are printed by exposure. FIG. 1B is a diagram showing an original film OF including an original image OI. On the printing plate PP, four identical images Ia-Id of the original image OI are printed. The images Ic and Id are oriented inversely of the images Ia and Ib and are complementary to the same.
One type of automatic photocomposer is provided with an automatic masking device for masking areas other than the image areas in exposing operation. FIG. 1A shows a window W of the automatic masking device for exposing the image Ia. The automatic masking device has four masking plates at its four sides respectively, which move to change the shape and size of the window W. The window W is therefore a rectangle. The area of the photosensitive material within the window W is to be exposed while the other area is masked.
When the images Ia and Ic are complementary to each other, as shown in FIG. 1A, the window W includes a partial area Rc1 of the image Ic in some cases. If the image Ia is printed by exposure while the window W is set as shown in FIG. 1A, the partial area Rc1 is also exposed while the image Ic is to be printed later. Incidentally, when a printing plate is made from a photosensitive material of positive type, printing ink is not put on the exposed areas, but is put on non-exposed areas. Accordingly, since the partial area Rc1 is exposed along with image Ia, printing ink is not put on the partial area Rc1 even though the image Ic is printed later.
This problem is not solved by forming the window W to be a rectangle circumscribed about an image in the case where circumscribed rectangles of plural images overlap each other. In order to cope with the problem, in the prior art, a mask made of a peel film (hereinafter referred to as "film mask") FM is prepared and overlaid on the original film OF so that the film mask FM masks the partial area Rc1 (FIGS. 1A and 1B).
However, the masking part of the film mask FM and its position on the original film OF are required to be changed according to the shape and the arrangement of the images Ia-Id. Therefore a considerably skilled worker is needed to decide the masking part and its position on the original film OF.